I have an old aluminum mast that is bent, but has a good main halyard installed. On one end, the shackle is permanently attached to the halyard (halyard goes through a hole in the shackle, and there is a ball on the end of the halyard), and on the other end is a loop. I am buying a new, straight mast that has no halyard attached. My question is this: how do I get the main halyard off of the old mast and onto the new one? I want to avoid drilling out and replacing the rivets if possible. Has anyone done this before?

The only idea I have is trying to break off the part of the loop end that keeps the loop shaped (I think it's called a thimble, maybe?), so I would instead have just a loop of wire. I could then squeeze the loop end so that it would fit through the pulley system and come out the other side. Are there any concerns with doing this? For example, does the thimble add necessary reinforcement to the loop, or will the halyard function properly without it?

The only idea I have is trying to break off the part of the loop end that keeps the loop shaped (I think it's called a thimble, maybe?), so I would instead have just a loop of wire. I could then squeeze the loop end so that it would fit through the pulley system and come out the other side.

You've more-or-less come up with the proper way to do this.

Cut off the eye as close to the crimping sleeve as you can. Remove from old mast. Install in new mast. Put in a new loop (without the thimble) so you can tie the line portion of the halyard to it, and make the change in the future without further shortening the wire halyard.

For anyone who looks here in the future, I was able to do this without cutting the halyard and installing a new loop. I just bent the thimble enough to get it out of the wire loop, then pulled the wire loop through the mast head (since it was just a wire loop at this point, I could get it through). Then installed halyard onto new mast. I was even able to put the thimble back on after I was done, although I am sure this is not necessary.